1. Field of the Invention
The present invention relates to a semiconductor device and particularly to a light-emitting device fabricated on a III-N compound semiconductor substrate, specifically, a GaN compound semiconductor substrate.
2. Description of the Background Art
GaN compound semiconductors have been employed or studied for light-emitting devices and high-power devices due to their specific characteristics. For example, it is technically possible to produce and utilize light-emitting devices in a wide emitting light range of violet to reddish yellow by adjusting the composition thereof.
Recently, utilizing the characteristics of the GaN compound semiconductors, blue light-emitting diodes and green light-emitting diodes have been put into practical use, and blue violet semiconductor lasers have been developed.
In order to fabricate a GaN compound semiconductor film, a substrate is used which is formed of sapphire, SiC, spinel, Si, GaAs and the like. If sapphire is employed for the substrate, prior to deposition of the GaN film by epitaxial growth, a buffer layer of GaN or AlN is formed at a low temperature of approximately 550xc2x0 C. in advance. After this step, the substrate is heated to a high temperature of approximately 1050xc2x0 C. to achieve epitaxial growth of the GaN compound semiconductor film. It is known that this process can provide a structurally and electrically superior crystal having a good surface condition.
It is also known that if SiC is employed for the substrate, a thin AlN film is advantageously used as a buffer layer at a growth temperature for epitaxial growth. However, the use of other materials than the GaN compound semiconductor as the substrate can cause a large number of defects in a resultant GaN compound semiconductor film, due to the differences in thermal expansion coefficient and lattice constant between the grown GaN compound semiconductor film and the substrate. These defects are classified into edge dislocation and screw dislocation, and the total density of the defects can amount to approximately 1xc3x97109 cmxe2x88x922 to 1xc3x971010 cmxe2x88x922. These defects are known to trap carriers and accordingly degrade electrical characteristics of the prepared films, and further to shorten the life of a laser to which a great amount of current is applied.
Thus, studies have been conducted for reducing these defects and enhancing electrical characteristics of the semiconductor to be prepared. For example, a technique has been developed in which on a GaN film grown by metal organic chemical vapor deposition (MOCVD) and the like, a thick GaN film is deposited by hydride vapor-phase epitaxy (H-VPE) and the like using a mask of SiO2, tungsten and the like for the purpose of preventing increase of defects such as dislocation, and the produced thick film is used as a substrate for forming a light-emitting device thereon.
However, characteristics of an n-type electrode on such a GaN substrate have not been clarified. The inventors of the present invention have found that an n-type electrode of Ti/Al or the like formed on a Ga-terminated surface of a GaN substrate has a strong tendency to exhibit Schottky characteristics. The inventors have considered that carbon (C) and the like are likely to be coupled to dangling bonds of Ga on the Ga-terminated surface. If an n-type electrode of Ti/Al or the like is formed on the Ga-terminated surface in the presence of C, a barrier layer can be produced and accordingly the electrode may exhibit Schottky characteristics. On the other hand, a film of Ni, Pd or the like constituting a p-type electrode can incorporate therein carbon (C) and the like to reduce the formation of the barrier layer. This is considered as one of the reasons for relative easiness of achieving ohmic characteristics by the p-type electrode.
In order to produce an n-type electrode of Ti/Al or the like with ohmic characteristics on the Ga-terminated surface of the GaN substrate, some steps are required, specifically including the steps of cleaning the substrate surface by hydrochloric acid and the like and annealing for producing alloy after fabrication of the electrode in order to form an intermediate product between GaN and Ti being therewith for reduction of the barrier layer. Specific contact resistance of the n-type electrode is still high even if those-steps are added.
One object of the present invention is to provide a technique of fabricating an n-type electrode in a semiconductor device structure employing a nitride semiconductor substrate such as a GaN substrate to give ohmic characteristics, without the steps of surface processing and annealing as described above.
Another object of the invention is to provide a nitride semiconductor device, particularly a light-emitting device, with a low specific contact resistance of an n-type electrode.
Still another object of the invention is to provide a nitride semiconductor device, particularly a light-emitting device, having a low threshold voltage or a low threshold current density.
The inventors have found that the ohmic characteristics can easily be obtained by fabricating an n-type electrode on an N-terminated surface of a nitride semiconductor. Further, the inventors have clarified the relation between the concentration of impurities added to a nitride semiconductor substrate and the specific contact resistance of the n-type electrode. Regarding light-emitting devices, particularly laser diode devices, the inventors have further clarified the relation between the concentration of impurities added to the nitride semiconductor substrate and threshold voltage as well as the relation between the concentration of impurities added to the nitride semiconductor substrate and threshold current density, and accordingly found a proper concentration of impurities which provides a low specific contact resistance, a low threshold voltage or a low threshold current density. The present invention has been made based on the above findings.
According to the present invention, a III-N compound semiconductor device is provided. The semiconductor device has an electrode on a nitrogen-terminated surface of the III-N compound semiconductor substrate. Specifically, the semiconductor device according to the present invention includes a III-N compound semiconductor substrate, a plurality of III-N compound semiconductor layers formed on the semiconductor substrate, and an n-type electrode and a p-type electrode for applying voltage to the semiconductor layers formed on the semiconductor substrate, wherein the semiconductor substrate is of n-type and the n-type electrode is formed on a nitrogen-terminated surface of the semiconductor substrate.
FIG. 23 illustrates a Ga-terminated surface and an N-terminated surface of GaN grown on the (0001) plane of a seed substrate, showing the seed substrate denoted by 2301, a buffer layer 2302, the Ga-terminated surface denoted by 2303B, the N-terminated surface denoted by 2303C, Ga atoms 2304 (represented by circle) and N atoms 2305 (represented by filled circle). As seen from the drawing, N atoms 2305 are predominantly projecting from N-terminated surface 2303C while Ga atoms 2304 are predominantly projecting from Ga-terminated surface 2303B.
The N-terminated surface and the Ga-terminated surface with respect to the (0001) plane of a GaN crystal can be defined here as follows. When the crystal having the exposed N-terminated surface is soaked for three minutes in an aqueous NaOH solution of 1.8 M at room temperature, its surface conditions change and hillocks of approximately 50 nm in size disappear. After such etching, roughened surface can also be observed by using atomic force microscopy (AFM), for example, at a region of 50 xcexcm in size. Such features can be shown by the surface in which N atom comprises at least 60% of the terminated atoms, and such a surface is herein referred to as N-terminated surface. On the other hand, the Ga-terminated surface has the feature that its conditions hardly change through the same processing, and after the etching almost no change can be observed by using AFM at a region of 50 xcexcm (see Appl. Phys. Lett. 71, 2635 (1997) for example). The surface in which Ga atom comprises at least 60% of the terminated atoms can show such a feature, and is herein referred to as Ga-terminated surface. Regarding the III-N compound semiconductor, the surface, in which N atom comprises at least 60% of the exposed terminator atoms and which has the feature that it can be easily roughened by a certain etching, can be called N-terminated surface. On the other hand, the surface, in which group III atom comprises at least 60% of the exposed terminator atoms and which has the feature that it can hardly change after a certain etching, can be called group III atom-terminated surface.
The difference in polarity (difference in terminator atom) may non-destructively be determined or evaluated by using reflection high electron energy diffraction (RHEED) (see for example Appl. Phys. Lett. 72, 2114 (1998)) or coaxial impact-collision ion scattering spectroscopy (CAICISS), instead of etching.
III-N compound semiconductors include for example GaN, AlN, AlxGa1xe2x88x92xN (0 less than x less than 1), InN, InxGa1xe2x88x92xN (0 less than x less than 1), InxGayAl1xe2x88x92xxe2x88x92yN (0 less than x less than 1, 0 less than y less than 1) and the like. Specifically, in the present invention, III-N compound semiconductor Containing Ga, i.e., GaN compound semiconductor is preferably used.
Typically, in the present invention, the concentration of n-type impurities in the semiconductor substrate is in the range of 1xc3x971017 cmxe2x88x923 to 1xc3x971021 cmxe2x88x923. Preferably, the concentration of n-type impurities in the semiconductor substrate is in the range of 1xc3x971017 cmxe2x88x923 to 1xc3x971019 cmxe2x88x923. In these ranges, the concentration of n-type impurities may be constant or vary in the direction of the thickness of the substrate.
In the present invention, the n-type impurity concentration in the semiconductor substrate may be invariable or variable in the direction of the thickness of the semiconductor substrate. If the n-type impurity concentration varies in the thickness direction, the semiconductor substrate preferably includes at least a first portion forming a nitrogen-terminated surface and having a first concentration of n-type impurities and a second portion having a second concentration of n-type impurities lower than the first n-type impurity concentration. If the concentration of n-type impurities varies in the thickness direction of the substrate, the first n-type impurity concentration of the first portion is preferably at least 3xc3x971018 cmxe2x88x923. The first portion preferably has a thickness from 0.05 xcexcm to 50 xcexcm.
If the concentration of n-type impurities varies in the thickness direction of the substrate, preferably a plurality of semiconductor layers are formed on the second portion having its n-type impurity concentration lower than the first n-type impurity concentration. In this case, preferably the first portion also has a thickness from 0.05 xcexcm to 50 xcexcm. In addition, the first portion preferably has the first n-type impurity concentration of at least 3xc3x971018 cmxe2x88x923.
The semiconductor device according to the present invention is typically a light-emitting device.
According to the present invention, another semiconductor device is provided. The semiconductor device includes a III-N compound semiconductor substrate, a plurality of III-N compound semiconductor layers formed on the semiconductor substrate, and an n-type electrode and a p-type electrode for applying voltage to the semiconductor layers formed on the semiconductor substrate, wherein the semiconductor substrate is of p-type, the uppermost layer of the semiconductor layers has a nitrogen-terminated surface, and the n-type electrode is formed on the nitrogen-terminated surface. The III-N compound semiconductor is typically GaN compound semiconductor. In this case, the p-type electrode is preferably formed on a Ga-terminated surface of the semiconductor substrate. The semiconductor device according to the present invention is especially applicable to a light-emitting device.
According to the present invention, a further semiconductor device is provided. The semiconductor device includes a III-N compound semiconductor substrate, a plurality of III-N compound semiconductor layers formed on the semiconductor substrate, and an n-type electrode and a p-type electrode for applying voltage to the semiconductor layers formed on the semiconductor substrate, wherein the semiconductor substrate is of n-type, the n-type electrode is formed on a nitrogen-terminated surface of the semiconductor substrate, and the concentration of n-type impurities in the semiconductor substrate varies in the direction of the thickness of the semiconductor substrate. The semiconductor substrate is composed of a first portion forming the nitrogen-terminated surface and having a first average concentration of n-type impurities and a second portion having a second average concentration of n-type impurities lower than the first average concentration of n-type impurities. The first average n-type impurity concentration is at least 3xc3x971018 cmxe2x88x923 and the second average n-type impurity concentration is at most 3xc3x971018 cmxe2x88x923. The semiconductor layers are formed on the -second portion. The first average n-type impurity concentration is preferably in the range of 3xc3x971018 cmxe2x88x923 to 1xc3x971021 cmxe2x88x923. The second average n-type impurity concentration is preferably in the range of 1xc3x971017 cmxe2x88x923 to 3xc3x971018 cmxe2x88x923. More preferably, the first average n-type impurity concentration is in the range of 3xc3x971018 cmxe2x88x923 to 1xc3x971019 cmxe2x88x923. The III-N compound semiconductor is preferably GaN compound semiconductor. The semiconductor device according to the present invention is especially applicable to a light-emitting device.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from following detailed description of the present invention when taken in conjunction with the accompanying drawings.